when the performance curve rates it at 1000rpm that is alternator rpm
not engine rpm
usually the alternator is ratio'd up to around 2.5 to as much as 3 : 1
so if the engine is idling at 650 it is turning the alternator well over cutin and it should be charging enough to cover all oem loads and charge a good battery.
nothing wrong with a higher output alternator, but i wouldn't automatically jump up to one if i didn't have a need for it.
the 3g alternator was used by ford on their L8000 with 8.3 liter
cummins, that was the biggest joke i ever saw
they engineered it with a single 8 groove serp to drive everything
including the hd air clutch fan, which over time with it cycling on
and off again would over stress the rear brg of the alternator and spit it out on the ground.
never had a new alternator from ford last over 6 months and even my best rebuilder could not make one to last over 8 months.
finally convinced the customer to change the mount brkt to a large frame leece neville and ended the issue.
car manufactures should not be allowed to build heavy trucks!
i digress,
for light duty trucks, the 3g and 4g are more than adequate, certainly
better than the delco cs130 when it comes to service/repair
bob g